A Few Plaster Questions...

Discussion in 'FAQs' started by Lighthorseman, Mar 3, 2003.

  1. Lighthorseman

    Lighthorseman Active Member

    Well, having read Shamus' rockwork article, I'm all pumped up and ready to give it a shot.

    My questions are twofold;
    --Where does one get "undercoat plaster"? My local home-improvement megastore (Revy) carries only basic plaster, such as plaster of paris (GAWD knows how old that stuff is...), and the "Pollyfilla" type wall-patching stuff.

    --What about tinting it? I really like the way Shamus' plaster starts with a basic earth colour. I've heard that tinting plaster will slow the setting time, sometimes to the point where it won't set at all. Any truth to this? What sort of tints should be used?

    My rockwork so far has consisted of WS rock molds. I used old plaster of paris, just because I had it at hand. This stuff just seems to suck up wash after wash after wash, and seems as though it likely won't stand up to much abuse.

    Anyone have hints or ideas?:confused:
  2. shamus

    shamus Registered Member

    Hi Steve,

    Adding emulsion paint to the undercoat-plaster will only add around ½ more to the hardening time. No truth at all about it not going off hard Steve, I nearly always add colour to it. Have done for many years and it always goes rock hard.
    As for the colour I did mention an earth colour. Household emulsion paint is used.
    I get my plaster at various places in the UK, “Homebase” – “Do-it-all” etc.

    Plaster of Paris goes off far too quick and is brittle unless used with a gauze of some kind similar to plaster bandages from a hospital.

  3. Woodie

    Woodie Active Member


    I first put a layer of plaster bandage over everything. Real cheap from the hobby shop. Just cut pieces off the roll, soak for 10 secs, then put in place.

    Next, I just use plain plaster of paris (or any powder plaster from the harware shop). Mix, as suggested, and then for, say, every "breakfast bowl" full you make up, I add in 1/2 teaspoon of elcheapo artist acrylic paint. ($5 for a "coffee cup" sized tub). You need to stir this in well, as it seems the plaster grains need to "burst" or something, to activate the paint colour.

    I use yellow ochre as my base plaster tint, as I want sandstone for my base. I then get a 1" paintbrush and paint it on over the plaster bandage, lump a bit more on where fit, and cut/mould to shape.

    Let this nearly set hard. (bout 1 1/2 hours) so it still just damp to touch.

    I then make up a wash of Red Oxide (artist paint) with 1/10 teaspoon paint to cup of water. Use a 2" paintbrush and dab liberally all over the plaster. Yes, let it run down the crevices to represent water runoff.

    Let dry/soak in.

    Repeat with Burnt Umber or Sienna. ( a little less liberally)

    Let dry/soak in.

    Repeat with charcoal black (a lot less liberally)

    Let dry/soak in.

    then apply ground cover.

    You don't have to be too accurate, or careful with the daubing. Just "splodge" it everywhere. Don't worry if it "pools", just let it dry off/soak in.

    Of course, these colours represent weathered sandstone. Take a close look at your prototype rock, and pick appropriate colours for your initial tinting, and, then washes. For granite? Maybe a light grey, but still use the Burnt umber and black for the washes.

    Anyway, have a little experiment, and let us know how it goes.

    Good luck. :)

    There are pics of building mine on my website. (click below and look at the "construction" pics.)
  4. 60103

    60103 Pooh Bah

    get plastered

    undercoat plaster must be a U.K. term. I think he may mean wall plaster. Given your choices, I would go with the Polyfilla. One really useful characteristic you want in your plaster is no shrinkage on setting.
    If you have friends who are doing walls, task them for some "mud" or "drywall compound", which is a bucket of pre-mixed plaster, used for smoothing the joints in drywall. I think it has a small shrinkage factor.
  5. shamus

    shamus Registered Member

    Re: get plastered

    Here in the UK, Wall plaster is called "Finishing plaster" and is not "Undercoat plaster". - Undercoat plaster is used to fill up big holes in walls, then the finishing plaster is used to smooth it all out, leaving a nice clean wall.

    To use polyfilla, would cost quite a bit of cash in relation to the undercoat plaster. I buy 56lb bags for £3-50p. Polyfilla for 1/2lb cost me around £1-50.

  6. aartwmich

    aartwmich Member

    I think the kind of plaster Shamus uses might be known in the states as 'brown coat' and you may not be able to find it at the HD types of stores.

    I think it's the coarse plaster used as the base coat for 'lath and plaster' walls which are nowadays only found in old old houses. I don't think it's the same stuff used to repair 'wallboard' type walls.

    I have an aquiantance who does restoration work on said old houses, I will try to get ahold of him and ask where we might find it here in the states.

    Another question tho...what the heck is household 'emulsion' paint?
  7. WildCat

    WildCat New Member


    I use Polyfilla from Home Depot in Calgary. About $22.00 CDN a large bag. Same for Edmonton. Works great but the amount of water to plaster makes a difference. Go by the instructions for W.S. rock molds. Using paper towels or newspaper, I find making the first coat really wet and laying it on the foundation works well as it does not tear apart so much. Then go back after it is dry and add a layer of drier mix to it. I did not like the result of mixing color in. Just paint it when dry with an earth color so the white is not showing. Then scenic with ground foam, etc.
    Hammer up any leftover plaster into rocks or stones. Color them with an acrylic or W.S colors to use later.
  8. kettlestack

    kettlestack Member

    Up till recently I used Pollyfiller until I discovered Sculptamold. Now that I've run out of Sculptamold I'm back to Pollyfiller.
    But now Shamus has given me the real "gen" :) .... thanks Shamus. I'm now on the lookout for Undercoat Plaster but I'll read the info on the bags before I buy it.

    One interesting thing about Sculptamold.... If after it has hardened and you thourouly wet it to paint with poster colours or take glue for groundcover, it softens again almost to the point it becomes reusable!!!

    On the subject of posterpaint, it is basically a powder pigment in water when you buy it. If you try overpainting it or "wetting" it again your previous work will run leaving it paler where it had been previously applied. In some circumstances, this is actually an advantage as it's rare to have large areas in nature all the same colour.

  9. jimnrose

    jimnrose Member

    In New England, we used 'scratchcoat' also called 'undercoat' plaster as the initial layer of plaster applied to 'blue board' sheetrock. It can be purchased at masonry supply stores. I'm still working on my rock formations and have used hydrocal, scratchcoat and finish plaster. As Shamus says, the scratchcoat is best because it has a longer cure time and yields a rough finish which can be further agrivated by adding baking soda (intended to entend the cureing time). I have used the rubber rock molds for some of the formatiions but am still trying to develop the 'free hand' sculpturing the Shamus is demonstrating. I'm not home, at this time, otherwise I'd identify the plaster manufacturer.
    take care, Jim
  10. aartwmich

    aartwmich Member

    Jim pretty much said the same thing my aquaintance said...he didn't know the manufacturer either..he hires out that part of the job and didn't really know as much as I thought he might.

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